linux - Dual boot Arch and Windows 10 with GRUB

I've been using Ubuntu for a while and recently decided to start using Arch. I have both a 120GB SSD and 1TB HDD in my system. When installing Arch on my SSD, I created partitions for /boot /home / (root) as well as a swap partition. I also have Windows 10 installed on my HDD using the default partitions. I would like to be able to dual boot between the 2 operating systems. I installed GRUB to my SSD on /dev/sda, but now when I boot into GRUB, I only see the option to boot into Arch, not Windows. I was wondering how I could boot into Windows via GRUB.

I have a default "/etc/grub.d/40_custom" file:

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries
.  Simply type the
# menu entries you want to add after this comment.  Be care
ful not to change

# the 'exec tail' line above.

When I run "lsblk", I get:

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 111.8G  0 disk 
├─sda1   8:1    0   200M  0 part /boot
├─sda2   8:2    0    12G  0 part [SWAP]
├─sda3   8:3    0    25G  0 part /

└─sda4   8:4    0  74.6G  0 part /home
sdb      8:16   0 931.5G  0 disk 
├─sdb1   8:17   0   499M  0 part 
├─sdb2   8:18   0   100M  0 part 
├─sdb3   8:19   0    16M  0 part 
└─sdb4   8:20   0 930.9G  0 part

Running "fdisk -l" gives me:

Disk /dev/sdb: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 47A5839B-C531-4BEE-A083-BD0C5CF4524A

Device       Start        End    Sectors   Size Type
/dev/sdb1     2048    1023999    1021952   499M Windows rec
/dev/sdb2  1024000    1228799     204800   100M EFI System

/dev/sdb3  1228800    1261567      32768    16M Microsoft r
/dev/sdb4  1261568 1953523711 1952262144 930.9G Microsoft b


Disk /dev/sda: 111.8 GiB, 120034123776 bytes, 234441648 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x1c797ba1


Device     Boot    Start       End   Sectors  Size Id Type
/dev/sda1           2048    411647    409600  200M 83 Linux
/dev/sda2         411648  25577471  25165824   12G 83 Linux
/dev/sda3       25577472  78006271  52428800   25G 83 Linux
/dev/sda4       78006272 234441647 156435376 74.6G 83 Linux

Any help would be appreciated, thanks!